"Would it even make sense to tell a mother to choose which daughter she would rather kill?"
Read Sophie's Choice
didn't they have a choice to tell the Duke's people that they would rather just be poor than take their money?
I assume the Duke's people ruined her father's business, so she will probably end up having to get revenge on them.
I doubt killing the Duke would put her in a good position, even if it is in self-defense...but death is death